1) Field of the Invention
The present invention relates to a method of manufacturing an optical waveguide, such as an optical fiber, having lower density of structural defects.
2) Description of the Related Art
It is well known that light absorption occurs if any structural defect exists in a portion through which light propagates (hereinafter, “light propagating portion”) of an optical waveguide. It is also known that if hydrogen molecules diffuse into the structural defect, the structural defect and the hydrogen molecules react with each other, which causes an increase in a transmission loss in the optical waveguide with time.
Japanese Patent Application Laid Open Publication No. 2002-148450 discloses a prior art for processing an optical fiber, which is an optical waveguide, to reduce the transmission loss. Particularly, the optical fiber is exposed to an atmosphere containing deuterium molecules (D2) before the optical fiber is used.
The prior art includes a step of exposing the optical fiber to mixed gas containing deuterium molecules, and causing the deuterium molecules to react with the optical fiber while the deuterium molecules are made to contact the optical fiber. The method also includes a step of removing the deuterium molecules from the optical fiber in a neutral atmosphere, i.e., in the air or in a nitrogen atmosphere. Treatment of the optical fiber using this method before it is used allows reduction in the increase in the transmission loss that occurs in the optical fiber caused by hydrogen with time.
In the prior art, the optical fiber is exposed to the mixed gas containing deuterium molecules. More specifically, the optical fiber is exposed to the mixed gas containing 0.01% to 100% of deuterium molecules, preferably, 0.5% to 2% of deuterium molecules at 20° C. to 40° C. for 1 day to 2 weeks, preferably, 3 days to 10 days. Thereafter, the optical fiber is left standing in the air or the nitrogen atmosphere for 1 week to 3 weeks, and then the deuterium molecules are removed from the optical fiber.
In other words, in the prior art, even if the treatment is performed under favorable conditions, the treatment requires at least 10 days. Moreover, the optical fiber is treated one line by one line (one bobbin by one bobbin) in a treatment vessel where the treatment is performed. Therefore, it is quite difficult to reduce the treatment time per bobbin of optical fiber.
In the prior art, the optical fiber is exposed to the mixed gas containing deuterium molecules for 1 day to 2 weeks (hereinafter, “exposing step”), and then the optical fiber is left standing in the air or the nitrogen atmosphere for 1 week to 3 weeks and an excessive amount of the deuterium molecules is removed from the optical fiber (hereinafter, “removing step”). More specifically, at the exposing step, deuterium molecules are caused to diffuse into inside of the optical fiber (hereinafter, “diffusing step”) while the optical fiber is exposed to the deuterium molecule-contained atmosphere, and the deuterium molecules are caused to react with the structural defects (hereinafter, “reacting step”). In other words, the exposing step, the diffusing step, and the reacting step are concurrently performed.
During the exposing step, the deuterium molecules continue diffusing into and penetrating the inside of the optical fiber through the periphery of the optical fiber, and even if the deuterium molecules finish reacting with the structural defects that exist in the light propagating portion, an excessive amount of deuterium molecules continues to be supplied to the light propagating portion. The excessive amount of deuterium molecules causes an increase in loss at a wavelength of about 1.7 micrometers. Broad absorption of the excessive deuterium molecules exerts influence over transmission characteristic at a wavelength of 1.5 micrometers that is a public-communication wavelength range. Therefore, the excessive deuterium molecules need to be removed, but in the prior art, the process extending over 1 week to 3 weeks is required only for the removal.
According to the experiments conducted by the inventors of the present application, it became clear that if the treatment is performed under an atmosphere that contains 100% of deuterium molecules, the exposing step can be performed generally in several hours, while the diffusing step and the reacting step require at least about 10 hours at a temperature ranging from 20° C. to 25° C. when a single mode optical fiber having an outer diameter of 125 micrometers is used. Of the two steps, the diffusing step is dominant, and the time required for reaction is extremely short.